1. Field of the Invention
The present invention relates to a producing method of a sealed battery. More specifically, the present invention relates to a producing method of a sealed battery including a current interrupt device (CID) in a battery case.
2. Description of Related Art
Sealed secondary batteries might experience abnormal increase in internal pressure over their normal ranges due to some abnormalities. To cope with this, there have been provided secondary batteries including CIDs that operate at higher internal pressures than normal internal pressures of the secondary batteries.
Some CIDs of secondary batteries include inversion plates made of metal plates each of which has a circular outer shape, and is provided at its center portion with a convex portion that is convex toward one side of front and back surfaces of the metal plate. Specifically, a central convex portion of an inversion plate of a CID is configured to be in contact with an internal terminal connected to an electrode body inside a secondary battery if an internal pressure of the secondary battery is in a normal state. Accordingly, if the internal pressure of the secondary battery is in a normal state, a current path at a position of the CID is put in a connected state.
Meanwhile, if the internal pressure of the secondary battery is increased up to its operating pressure, the inversion plate is inversely deformed due to the internal pressure so as to generate a gap between the convex portion at the center of the inversion plate and the internal terminal. Specifically, if the internal pressure of the secondary battery becomes increased up to its operating pressure, the CID operates so as to cut off the current path at the position of the CID. The secondary battery with the CID in operation is configured to prevent further charging and discharging.
An example of prior art references about the secondary battery including the above CIDs may be Japanese Patent Application Publication No. 2015-125798, for example. JP 2015-125798 A describes a secondary battery including a CID configured such that an inversion plate having a circular outer shape and a convex portion formed at its center is fitted into a recessed portion in a circular shape of a rivet, and the plate and the rivet are then joined together. The inversion plate and the rivet are joined together by welding an outer edge of the inversion plate.
FIG. 14 shows a sectional view in a radial direction of a rivet 90 and an inversion plate 80 fitted in a recessed portion 91 of the rivet 90. Each of the recessed portion 91 and the inversion plate 80 is formed in a circular shape. The inversion plate 80 includes a circumferential edge 83 located radially outward, and a central portion surrounded by the circumferential edge 83, and this central portion becomes convex upward. While a first surface 81 faces upward, the inversion plate 80 is inserted until a second surface 82 of the circumferential edge 83 comes into contact with a bottom surface 93 of the recessed portion 91. With the state shown in FIG. 14, generally, the inversion plate 80 and the rivet 90 are joined together by laser beam welding that radiates a laser beam from above to a boundary between the inversion plate 80 and the rivet 90 at which a side surface 84 of the inversion plate 80 faces an inner wall surface 92 of the recessed portion 91. In the laser beam welding, the boundary between the inversion plate 80 and the rivet 90 is scanned by at least one round with the laser beam. This is for the purpose of joining the inversion plate 80 and the rivet 90 together with no gap therebetween.
As shown in FIG. 14, the side surface 84 of the inversion plate 80 is often so formed as to be an inclined surface inclined in a direction further apart from the inner wall surface 92 of the recessed portion 91 as this side surface is located at a position closer to the bottom surface 93 of the rivet 90. This is because the side surface 84 of the inversion plate 80 is set to be an inclined surface as shown in FIG. 14 so as to set a distance between the inner wall surface 92 of the recessed portion 91 and the side surface 84 of the inversion plate 80 to be narrower as this distance is located at an upper position toward the laser beam radiation side, thereby increasing a joint strength therebetween.
Unfortunately, if the side surface 84 of the inversion plate 80 is set to be an inclined surface inclined in a direction further apart from the inner wall surface 92 as the side surface 84 is located at a position closer to the bottom surface 93, as shown in FIG. 14, a space Z is formed between the inversion plate 80 and the recessed portion 91 of the rivet 90. Specifically, the space Z is formed at a position surrounded by the inclined side surface 84 of the inversion plate 80, the inner wall surface 92 of the recessed portion 91, and the bottom surface 93 of the recessed portion 91. The space Z is formed along the side surface 84 of the inversion plate 80 by one round, generally in a ring form. This space Z in a ring form is a space that slightly opens only in a gap at an upper position between the side surface 84 and the inner wall surface 92, and is an almost closed space from the outside.
In the laser beam welding, the space Z in a ring form between the inversion plate 80 and the recessed portion 91 is gradually charged with a molten portion flowing therein continuously from a starting position of the laser beam radiation. Hence, a gas in the space Z is sent by the molten portion with which the space Z is charged along the space Z frontward in an advancing direction of the laser beam. This is because the space Z is an almost closed space from the outside.
As shown in a sectional view of the advancing direction of the laser beam L in FIG. 15, when the laser beam L goes around from a starting position S by one round, and then reaches the starting position S once again, the gas in the space Z is held in between a left end of a joined portion WA hardened at the starting position S and a molten portion M formed by the radiation with the laser beam L. If the side surface 84 of the inversion plate 80 is formed to be an inclined surface as shown in FIG. 14, the joined portion WA is likely to be formed at a position (upper position) located on the laser beam radiation side where the distance between the inner wall surface 92 of the recessed portion 91 and the side surface 84 of the inversion plate 80 is narrower. Hence, as shown in FIG. 15, the left end of the joined portion WA hardened at the starting position S is likely to have a shape projecting leftward as the left end is located at a position closer to the laser beam L radiating side.
Accordingly, subsequent to FIG. 15, along with the shift of the laser beam radiation position, the space Z gradually becomes a space completely closed from the outside by the molten portion M and the left end of the joined portion WA. In addition, the gas inside the completely closed space Z is gradually compressed in accordance with decrease of the space Z along with the shift of the laser beam radiating position. The gas inside the space Z is compressed, and also expands at the same time in accordance with increase in temperature due to the radiation with the laser beam L. Consequently, the enclosed gas might abruptly break the molten portion M, thus splashing a part of the molten portion M all over.